As a method for laying an optical fiber cable, a pressurized gas carriage system has been proposed in which an optical fiber unit composed of a single optical fiber or plural optical fibers is carried in a pipe made of synthetic resins, etc. by use of compressed air, etc. as disclosed in JP-A-59-104607 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). According to this cable laying system, parts of a pipe cable other than an optical fiber unit are produced in a factory and laid on buildings, etc., and an optical fiber unit is then laid to complete a communication cable in the final stage of construction.
In this system, optical fibers receive no outer force, such as tension, the optical fibers are not damages upon laying. Further, since the optical fiber unit is carried through a previously laid pipe by utilizing a compressed fluid, it can thread its way even on a complicated route. Furthermore, since optical fiber units can be exchanged or added after construction, maintenance of communication equipment is easy and the cost of cable laying is reduced.
On the other hand, with the recent rise of height of buildings, demands for communication cables to have flame spread resistance or surviving properties on fire have increased as pointed in Proceedings of The 1989th International Wire and Cable Symposium, pp. 301-305.
Hence, the inventors of the present invention previously proposed a flame-retardant pipe cable as described in JP-A-2-114219. The structure of the proposed pipe cable is shown in FIGS. 2(a) and 2(b), in which six pipes 11 each containing an optical fiber unit and high tensile body 12 are jacketed by outer sheath layer 10 comprising flame-retardant polyethylene, and thereby the pipe cable has improved flame retardance. Each pipe has a double-layered structure composed of flame-retardant polyethylene outer layer 13 and crosslinked polyethylene inner layer 14 as illustrated in FIG. 2(b).
However, even with such a structure, melting of the resins is hardly avoidable when heat above a certain level is applied for a long time, and outer sheath layer 10 and flame-retardant polyethylene layer 13 melt and run to cause the optical fiber units laid in the pipes to be exposed to heat and air and broken by combustion. Therefore, the conventional pipe cables are still insufficient in survivability on fire as communication cables.